This invention relates to a nonaqueous electrolyte battery provided with a cathode having an active material such as a metal oxide or sulfide, an anode comprising a consumable metal such as lithium, and a separator rolled into a spiral electrode unit.
In general, in a lithium battery containing a spiral shaped electrode unit within an exterior package, the anode is placed around the outermost periphery of the electrode unit to make sufficient use of the active cathode material. A nonaqueous electrolyte battery having the anode around the outermost periphery of the electrode unit is described in Japanese patent disclosures 53-29933 (1978) and 63-6988 (1988).
In a spiral electrode unit of this type, even after battery discharge, one turn of the outermost peripheral anode which faces the cathode on only one side remains only half consumed and unable to react further. This is because the anode thickness is designed for anode depletion at discharge resulting from both sides of the anode reacting with the cathode material.
Prior art batteries connected the anode terminal tab to the remaining one turn of the outer peripheral anode. For this reason, these batteries suffered from internal shorting problems when completely discharged by forced discharge. Namely, since the anode terminal tab was connected to the remaining outermost part of the anode, conduction occurred through the remaining anode when power was overdrawn from the battery, thereby exceeding the reaction limit of the active cathode material. This caused migration of the anode material to the surface of the cathode facing the remaining anode and deposition of the anode material on that cathode surface. Ultimately, the deposited anode material penetrated through the separator causing conduction between the cathode and the anode.
To deal with this problem, prior art nonaqueous electrolyte batteries had anodes made as thin as possible. This reduces the amount of active anode material remaining in the outermost turn of the anode after discharge and suppresses lithium deposition on the cathode.
However, although the thin anode battery can reduce lithium deposition on the cathode during complete discharge, anodes are easily broken during the electrode rolling process and manufacturability is degraded. Further, by reducing the anode thickness, the actual capacity of the anode is reduced bringing about a large performance degradation.
Batteries with both the cathode and anode made thin and the electrode area increased can reduce the amount of lithium deposited per unit area of cathode because the electrode surface area is large. However, to reduce lithium deposition per unit area, these batteries require large reductions in both cathode and anode thicknesses. With the opposing electrode surface areas increased more than necessary, internal resistance is reduced and large currents flow when the battery is shorted externally. Large currents cause battery overheating, and internal shorting due to separator melting becomes a concern.
The present invention was developed to solve the previously mentioned problems. It is thus a primary object of the present invention to provide a nonaqueous electrolyte battery that prevents anode migration to, and deposition on the cathode surface induced by conditions such as overdrawing power from the battery, by disconnecting the outermost turn of the anode remaining after discharge from the anode terminal tab.